If flying I'd opt for the new Ho Chi Minh route on Air Asia too. Your sons are quite old though and you have two months so there shouldn't be any problem travelling overland via Cambodia with them if you prefer. Maybe see Angkhor Wat and the Phnom Penh war museum (S21) en route. There's some information about the land border crossings here. Cambodia: Overland Travel and International Border Crossings
Be careful about visas though. You can get the Cambodian visa on arrival but the Vietnam visa needs to be obtained in advance AND you need to tell them the exact dates and points of entry and exit for Vietnam.

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“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.